Ultrasound imaging of various aspects of the heart is a well known diagnostic modality. Various systems have been designed which enable an ultrasound system to determine the volume of the left ventricle of the heart at various times during the cardiac cycle. Such systems require an ability to accurately identify the endocardium/blood boundary. In U.S. Pat. No. 5,195,521 to Melton, Jr. et al, assigned to the same assignee as this application, a majority vote circuit indicates, at each range along an ultrasound-scan line, when a majority of the signals for both a current scan line and two previous scan lines indicate that reflections of the transmitted pulses are from tissue or blood. In this manner, the tissue/blood interface is accurately determined and is less affected by noise.
U.S. Pat. No. 5,257,624 to Fraser et al describes a gain control circuit which enables the gain along one or more scan lines to be kept substantially constant--thereby enabling a more accurate tissue/blood boundary determination. The accuracy enhancement occurs due because the boundary judgement is based upon a discrimination in levels between return pulses, with tissue generally returning a higher level signal than blood. If the gain of return signals from a scan line vary significantly due to tissue attenuation, tissue/blood boundary determination is rendered substantially more complex.
In U.S. Pat. No. 5,322,067 to Prater et al, (assigned to the same assignee as this application) describes an improved technique for determining volumetric efficiency of the left ventricle. An ultrasound display of the left ventricle and surrounding tissue is obtained and the user traces a region of interest around the ventricle at the largest volume for which a volume determination is to be made. Each pixel of the ultrasound image, within the region of interest, is classified as a blood pixel or a tissue pixel. The area of blood pixels within each display frame is determined and the volume of the ventricle is calculated from the area of the fluid pixels within each segment of the region using the method of disks.
In U.S. Pat. No. 5,285,788 to Arenson et al., an ultrasound imaging system displays a B-mode image and superimpose thereon color Doppler information. The color Doppler information is displayed using a color map which indicates different accerations of a coronary myocardium.
The cardiologist is interested in being able to assess left ventricular function. Ventricular (i.e. endocardium) wall motion is an important aspect of the diagnostic information. The physician wishes to know how much each segment of the endocardium moved and when the individual segment moved with respect to other individual segments. The conditions being assessed are: akinesis (no movement), hypokinesis (partial movement), hyperkinesis (over-active movement), diakinesis (abnormal movement), and tardokinesis (delay of movement). In presently available cardiac ultrasound imaging systems, wall motion assessment by the physician may only be performed only on a qualitative (as contrasted to quantitative) basis. Such an assessment is difficult because the physician's eyes must essentially integrate the movement of each piece of the endocardium wall.